The present invention relates to cementing operations, and more particularly, to improved cement dispersants for cement compositions and methods of cementing.
Hydraulic cement compositions are commonly utilized in subterranean applications, including, but not limited to, well completion and remedial operations. For example, in subterranean well applications, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and/or positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like.
Dispersants are often used in cement compositions to obtain a desired set of rheological properties in and to facilitate mixing of the cement compositions. Adequately dispersed cement slurries generally exhibit improved fluid-loss control, can displace drilling mud more efficiently, and may be successfully mixed and pumped at higher densities with less frictional pressures. In addition, the lower viscosity often allows the cement slurries to be placed by turbulent flow. Turbulent flow characteristics are often desirable when pumping cement compositions into subterranean wells, as turbulent flow placement of cement may remove drilling fluids more efficiently from surfaces in the well bore. The inclusion of dispersants in cement compositions is also desirable in that the presence of the dispersants may facilitate the mixing of the compositions and reduce the requisite water. Reduced water content in cement compositions is often associated with improved compressive strength.
A number of dispersing agents have been utilized heretofore in cement compositions, particularly in cement compositions used for primary and remedial cementing in subterranean wells. Commonly these dispersants are sulfonic acid derivatives of aromatic or aliphatic hydrocarbons. One of the most common cement composition dispersants is a condensate of acetone, formaldehyde, and bisulfite. Such dispersants, however, may be problematic because of their possible adverse impact on the environment and failure to meet certain environmental regulatory requirements.
Other conventional dispersants that meet, or are exempt from, environmental regulations are available. These dispersants, however, have significant disadvantages. For example, lignosulfonate derived dispersants often retard cement setting, sometimes resulting in an overretardation of the cement composition. This problem is further amplified in cooler wells, such as those found in the North Sea. Set time retardation may also prolong the time spent waiting for the cement to set, contributing to increased operating costs. Additionally, lignosulfonate derived dispersants are a wood pulp by-product from the paper industry. As such, lignosulfonate derived dispersants suffer from quality control issues that stem from batch inconsistency.